In the existing mobile network, a large number of networks based on packet switch (PS) networks (such as Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and Universal Terrestrial Radio Access Network-High Speed Packet Access (UTRAN-HSPA)) are deployed, however, their coverage is not all-network or all-region. The signals of the PS network at some places would be very weak, then the transfer from the PS network to the conventional circuit switch (CS) network (such as UTRAN and Global System for Mobile Communications (GSM) Enhanced Data Rate for GSM Evolution (EDGE) Radio Access Network (GERAN)) needs to be provided to the user, especially when the user is during a session.
FIG. 1 is a transfer way recognized by the standard organization, and firstly there is a call between user equipment (UE) 1 and UE2 and it is a PS bearer based on an IP multimedia subsystem (IMS), the session of which is anchored in a service centralization and continuity application server (SCC AS). During the call, UE1 moves to an area with better coverage of CS network, at this moment, UE1 will send the received signal test report to a mobility management entity (MME) via the original PS network. When the MME knows that it has to transfer from PS network to CS network after receiving the test report, it sends a transfer request to one mobile switching center (MSC) which can cover UE1, and then the MSC sends the transfer request to the IMS; since the session is anchored in the SCC AS, the SCC AS will update the session with UE2 after receiving the transfer request. When the session update is completed, the MSC will return a transfer completion response to the MME, the MME returns the transfer completion response to UE1, and when receiving the transfer completion response, UE1 starts to disconnect from the PS network and establish a connection to the CS network via the MSC. During the entire transfer, the calling user and called user are not perceivable to the transfer.
As to an undergoing call, i.e. the calling party sends a call request and the called party is in the ringing state but has no reply, then the calling party can hear the customized alerting tone or ordinary ringtone. As to such a call, if the transfer condition is met, the transfer can also be carried out.
FIG. 2 is a flowchart of transferring an undergoing call origination request in the related art.
201: UE1 initiates a call origination request to the IMS network via a calling proxy call session control function (P-CSCF) or a calling serving call session control function (S-CSCF) to make a request for having a dialog with UE2;
202: when receiving the call origination request, the calling P-CSCF triggers a call to the calling serving call session control function (S-CSCF) according to the address of S-CSCF acquired when UE1 registers, then the S-CSCF triggers the call to the SCC AS; or when receiving the call origination, the calling S-CSCF triggers the call to the SCC AS;
203: the SCC AS anchors this call origination request and prepares for the subsequent transfer;
204: the SCC AS returns the call origination request to the calling S-CSCF;
205: the calling S-CSCF routes the call origination request to the called S-CSCF according to number of called user;
206: the called S-CSCF routes the call origination request to the called user UE2;
207: after receiving the call request, UE2 sends a 180 ringing to the calling user UE1 to indicate that the called UE2 has received the request;
208: at this moment, due to the change in location, UE1 receives the test report of the CS network indicating that the signals of the CS network are better and the dialog of the PS network is required to be transferred to the CS network;
209: an access layer network element (such as MME) notifies the MSC, then the MSC initiates a transfer request to the IMS network or initiates a transfer request to the IMS network via a media gateway control function (MGCF);
210: the calling I-CSCF sends the transfer request to the SCC AS;
211: after receiving the transfer request, the SCC AS updates one undergoing call of UE1 according to the contents in the request, which is mainly to update the media type and sending address of the calling UE1;
212: after receiving the update request, the called S-CSCF updates the undergoing call with UE1;
213: after having completed the update, UE2 sends an update completion reply to the called S-CSCF;
214: the called S-CSCF returns the update completion reply to the SCC AS;
215: after receiving the update completion reply of the called party, the SCC AS sends a transfer completion reply to the calling I-CSCF; and
216: the calling I-CSCF sends the transfer completion reply to the MSC or sends the transfer completion reply to the MSC via the MGCF.
By then, the transfer is substantially completed. However, during this transfer, the situation that the call has a customized alerting tone service is not taken into account, especially the situation that the customized alerting tone service is played in the way of early session.
When the customized alerting tone is played in the way of early session, its simplified procedure is as shown in FIG. 3.
301: UE1 initiates a call origination request with a session description protocol (SDP) request regarding to the normal media negotiation of call;
302: after receiving the call request, the calling IMS network element routes the call request to the called network side, and the called network routes the call request to a customized alerting tones application server (CAT AS) by way of the subscription information of the called user;
303: the CAT AS routes the call request to the called UE2;
304: UE2 returns a reply after receiving the call, and at the moment, the reply includes an SDP reply of UE2 on the normal media negotiation;
305: the CAT AS forwards the reply message of UE2 and inserts the SDP request of customized alerting tone session indicating the customized alerting tone service into the reply message;
306: the calling IMS network element forwards the reply message to the calling UE1;
307: UE1 replies the SDP reply of the customized alerting tone session according to the SDP request thereof so as to establish a SDP negotiation of a customized alerting tone media with the CAT AS;
308: also in step 306, UE1 also receives the SDP reply about normal media negotiation so as to establish a SDP negotiation of a normal media with UE2.
By then, the call is connected, UE1 can hear the customized alerting tone of the called user, and UE2 is in the ringing state.
It can be seen from the above call origination procedure that there are two SDP negotiations at the UE1 side, one is a SDP negotiation of the customized alerting tone media, which plays the customized alerting tone service, established with the CAT AS, and the other is a SDP negotiation of the normal media established with UE2.
If the transfer is carried out according to the procedure shown in FIG. 2, as to the situation that the MSC supports early session, when receiving the transfer request, the SCC AS will only update the SDP negotiation of the normal media, and the SDP negotiation of the customized alerting tone media will no longer be updated. As to one undergoing call origination, the calling user may have such experience: when the telephone is connected, the calling party hears the customized alerting tone played by the called party, at this moment, the location of the calling user changes and the transfer from PS network to CS network is required; after the transfer is completed, since the SDP negotiation of the customized alerting tone media is not updated, the customized alerting tone heard by the calling party will stop suddenly, and the normal ringtone will not be played, and at this moment, the calling party may deem that the call fails and hang up the telephone, thereby affecting the normal call.